Though President Donald Trump has already ramped up efforts for a 2020 reelection campaign, one of his frequent critics, Sen. Susan Collins (R-Maine), says it’s unclear whether he will be the GOP nominee.

“It’s too difficult to say,” Collins said in an interview with MSNBC on Monday when asked whether she thinks Trump will be the Republican Party’s nominee. Collins did not support Trump in 2016; instead, she wrote in the name of House Speaker Paul Ryan.

“That was very hard for me to do as a life-long Republican,” Collins said. Collins is one of a slew of critics Trump has in the GOP-controlled Senate. She cast one of the three key votes that sank Republicans’ seven-year effort to repeal and replace the Affordable Care Act.